This invention relates to charge transfer imaging cartridges for creating latent images on a dielectric for subsequent toning and transfer to a carrier. More particularly, the invention includes cartridges for creating the images and a method of making the cartridges.
The present invention is described herein with reference to an exemplary printer which utilizes a dielectric coated print drum. However, it will be clear to those skilled in the art that the present invention may also be used in combination with printers utilizing different configurations of image receiving surfaces, and indeed may be useful in machines other than printers.
There is an increasing need for peripherals which can accept a computer or word processor output and convert the output to an image on paper, commonly called a "hard copy". Typically such a peripheral is a printer which uses a charge transfer process similar to that described in U.S. Pat. No. 4,267,556 to Fotland and Carrish. This printer utilizes a combination of electrodes about a dielectric which can be controlled to place a charge on a drum coated for instance with aluminum oxide impregnated with a wax. In this way a latent image is built up corresponding to the image to be produced on the paper, and the latent image is then toned and transferred to the paper and fused. Should it be necessary to produce a second copy, the procedure is repeated to give as many copies as necessary. Further, it is possible to vary the image by electronic control so that parts of the image can be printed, or the complete image can be turned through 90 degrees with respect to the paper. These possible variations make such printers desirable equipment wherever hard copies of electronically generated information are required.
An example of cartridge construction is described in applicant's U.S. Pat. Nos. 4,679,060 and 4,745,421 both to McCallum et al. This cartridge includes a number of relatively thin planar structural layers and produces a charge transfer image by means of a charge generator in the form of a matrix of electrodes located on an inner surface of the cartridge. The charges generated by the cartridge are formed through the application of a high voltage alternating potential between two conductors, commonly referred to as driver electrodes and finger electrodes, separated by a solid dielectric. The finger electrodes are provided with a multiplicity of holes around the edge of which the charges are formed, and an extraction voltage pulse is supplied between the finger electrodes and the print drum to attract the charges to the dielectric surface of the drum. In order to create a dot image on the drum from any one hole, two potentials must be present simultaneously, that is, the discharge potential and the extraction potential. This permits dot matrix multiplexing with a minimum number of interconnections and pulse drive sources.
The cartridge described in this patent also describes a further screen electrode between the finger electrode and the drum which acts to provide better definition of the dot images.
In use, it has been found that the preferred material for the dielectric between the driver and finger electrodes is mica, especially Muscovite mica, H.sub.2 KAl.sub.3 (SiO.sub.4).sub.3, as it possesses the desirable qualities for a dielectric in such circumstances, namely: high dielectric strength, low dissipation factor, high dielectric constant, high corona resistance, and is translucent which facilitates the positioning of the various electrodes during manufacture of the cartridge.
Dielectric strength is simply the minimum voltage required to cause physical breakdown, for example, puncturing, of a insulating film of a given thickness. This is important in cartridges as the dielectric will have to withstand 2000 to 3000 Volts peak-to-peak at radio frequency, and the dielectric layer must be kept relatively thin for the formation of charge to occur. The dielectric strength of mica is in the range of 3000 to 6000 Volts/Mil.
The dissipation factor of a material can be stated in terms of the difference between the amount of energy required to charge a capacitor with the material between the plates and the amount of energy received in return when the capacitor is fully discharged. The difference, or energy losses, arise from both the inherent electrical resistance of the dielectric and from hysteresis effects, and result in heating of the dielectric. The dissipation factor of mica is normally 0.01 to 0.04.
For an insulating material, the dielectric constant (k) is defined as the ratio of the electrical capacity of a capacitor with that material between the plates to the electrical capacity of a similar unit with air between the plates. For all practical purposes, the dielectric constant of the dry air is taken as unity, and the dielectric constant of mica is in the region of 6.5 to 8.
The creation of charges at the finger electrodes takes the form of a corona discharge, which process includes the creation of substances which tend to degrade dielectric materials in addition to the degradation effects of the dielectric stress on the material. The corona resistance of a material is simply a measure of its ability to withstand this degradation.
While mica meets the desirable specifications, it suffers from a number of disadvantages. At present, mica is only available from a single source and continued reliable supply cannot be assured. Also, as mica is a naturally occurring material, there is only a finite reservoir available, and as the demand for such cartridges increases, this reservoir will become depleted. However, the main reasons for seeking an alternative to mica are its physical limitations. Mica is fragile and liable to cracking and must therefore be handled very carefully during shipping and at all stages of manufacture of the cartridges. Also, mica is only available in a limited range of sizes and this limits the possible physical dimensions and configurations of cartridges. This is a handicap now that there is a demand for longer cartridges to create wider images, and in an attempt to overcome this limitation some work has been done to develop modular cartridges made up of two or more shorter cartridges to be used in place of a single longer cartridge. In the event it has proven difficult to match the images created by adjacent cartridges evenly and it is of course more expensive to produce two or more shorter cartridges in place of one longer cartridge. Also, a demand has arisen for narrower cartridges. Previously, this has been impractical because mica is always sized to leave a width of mica between the edge of the mica sheet and the area in which uniform properties are required, because the breaking of the mica at the edges causes unpredictable discontinuities and cracking.
Initial investigations to locate an alternative dielectric were directed to glasses and glass ceramic dielectrics, which, while possessing many of the necessary properties outlined above, created other difficulties. High temperature dielectrics, requiring firing above 850.degree.C., required the provision of a ceramic substrate for the cartridge in place of the traditional epoxy substrate. The provision of the ceramic substrate made the cartridges prohibitively expensive. Lower temperature dielectrics, having firing temperatures around 600.degree.C., required the use of either a glass substrate, which was found to be fragile and to provide poor heat sink, or a porcelaine coated steel substrate, which while being inexpensive and easily formed has an uneven surface unsuited for use in a cartridge.
Other problems were encountered in placing the dielectric on the substrate using the preferred method of application, that is, screen printing, which resulted in large area defects. Also, plating the driver electrodes on porcelain was found to be difficult, and the plated driver electrodes also tended to react with the glass dielectrics.
Low temperature dielectrics were a generally more attractive alternative in view of the less arduous curing techniques required, which would permit continued use of the current manufacturing process, though initial tests with such commonly available low temperature dielectrics such as epoxies, phenolics and acrylics confirmed the commonly held view that low temperature dielectrics had poor corona resistance and would have a very short life span in a cartridge.
During testing of the various dielectrics it was discovered that corona resistance was significantly improved when partially assembled cartridges were tested. This was found to be the result of the presence of a silicone based adhesive on the surface of the dielectric which was subsequently removed during the further manufacturing process. This led to an investigation of silicones generally and further testing revealed that silicones, or polymeric organic siloxanes, exhibited high corona resistance.
A subsequent search located a number commercially available silicone modified polymers intended for other uses. Initial tests showed the materials to have the necessary corona resistance and the potential for use in place of mica.